I have encountered the same effect and found I could make it happen at will creeping down the driveway with the brakes cold. It is as if the pad to disc coefficient of friction is not stable, and it gets into a "stick-slip" effect, like a prop shaft on a ship running at near zero rpm.

In my 350SD I can get the brake force just right and have the automatic transmission push the car at a few inches per second and hold the brake so that the noise is virtually constant.

All that is amusing, but likely not a problem. I have put 100,000 miles on the car and not had any troubles with the brakes. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)